1. Field of the Invention
The present invention relates to a device and a method for the coherent combination of two optical beams transformed by optical components limited in energy and/or in average power and/or in peak power. Especially, the invention relates to a device and a method for the coherent combination of two optical beams amplified, for example, by two independent optical amplifiers. The invention also relates to a device and a method for the coherent combination of two optical beams spectrally broadened by two independent spectral broadening devices. The invention also relates to a device and a method for the coherent combination of two optical beams amplified and spectrally broadened, for example, by two non-linear optical amplifiers.
2. Description of the Related Art
It is searched to develop optical beams, and in particular energy laser beams with higher and higher peak power and/or average power while having an optical beam with an excellent spatial quality. It is also searched to develop reduced-volume devices based on the use of integrated optical components, such as an optical fiber amplifier, or a spectral broadening device based on a hollow capillary fiber, or on a large core (LMA or large mode area) fiber, or on small core fibers having particular dispersive properties. However, these optical components are limited in energy and/or in average power and/or in peak power. Different devices have been proposed to distribute the energy of a beam between several independent optical components, each optical component being limited in energy and/or in average power and/or in peak power. The difficulty is then to coherently recombine the different beams so as to preserve their spatial, spectral, temporal qualities, while increasing their energy, average power, peak power. In the case of the optical amplification, a first technique to obtain a high energy optical beam consists in amplifying a beam by two successive amplification stages. Starting from an oscillator generating initial pulses in the domain of 10 pJ to 1 μJ, a pre-amplifier allows obtaining pulses with intermediate energies of 10 nJ to a few μJ. A second optical power amplifier is necessary to reach the energies of 10 μJ to a few mJ required for most of the applications. However, the optical amplification in a non-linear optical medium is liable to generate non-linear effects (self-phase modulation . . . ) responsible for a deterioration of the optical quality of the amplified beam and more precisely of the temporal (ultra-short pulse) and/or spectral properties of the optical beam. Moreover, the high average power amplification may deteriorate the spatial properties of the beam (single-mode beam) due to the thermo-optical effects.
Another way to obtain a high energy and/or high power and/or high peak power beam is to use several sources or several independent amplifiers and to combine the beams coming from these different sources or from these different amplifiers. However, in order to preserve all the qualities of the laser beam, the recombination of the different beams must non only allow the spatial and temporal superimposition thereof, but also a coherent recombination, i.e. with a phase difference stable over time. The coherent combination of amplified laser beams is a very promising technique for the development of high energy and/or high average power and/or high peak power laser systems. However, the relative phases of different beams may fluctuate rapidly. The most difficult technical problem posed by the coherent recombination is to maintain a constant relative phase between different optical beams.
The coherent recombination of several optical beams has nevertheless been made by means of either passive or active devices.
The publication “Laser beam combining for High-Power, high-radiance sources”, of T. Y. Fan, IEEE Journal of selected topics in Quantum Electronics, vol. 11, n° 3, 2005, indicates the fundamental conditions required to perform a recombination of laser beams (control of the power, relative phase, polarization, amplitude and alignment of each beam to be combined) and describes different methods for the coherent combination of optical beams, in order to obtain a high average power beam with spatial, temporal, spectral and almost-ideal polarization qualities.
The patent document U.S. Pat. No. 5,307,369 (D. E. Kimberlin) describes a passive system for the coherent combination of two amplifiers placed inside a common resonant cavity divided into two sub-cavities by a semi-reflective mirror. This device is similar to a double optical counter-reaction oscillator, a part of the beam emitted by the first laser amplifier being injected in the sub-cavity of the second laser amplifier, and vice versa. The output combined laser beam is a result of multiple coherent reflections occurring in the laser cavity. This device allows doubling the average power of a continuous laser beam or of synchronized laser pulses emitted by the two lasers. However, the differences of optical paths between the two sub-cavities induce phase-shifts that limit the stability of the passive device and the output power.
The passive combination appearing limited, various active devices for the coherent recombination of optical beams have been proposed. The active recombination is based on a direct or indirect measurement of the relative phase between the optical beams to be combined and on the introduction of a phase-shift actively controlled by a feedback loop on each optical beam. An active device for coherent recombination generally takes a part of the beam before or after recombination to extract therefrom a measurement of the phase-shift between the optical beams and adapts in real time the relative phase on each beam by means of an acousto-optic modulator, a piezoelectric mirror, or by adjustment of the optical pumping power.
Thus, for the amplifiers, an active device for coherent combination is generally used [Wei Liang, Naresh Satyan, Firooz Aflatouni, Amnon Yariv, Anthony Kewitsch, George Rakuljic, and Hossein Hashemi, “Coherent beam combining with multilevel optical phase-locked loops,” J. Opt. Soc. Am. B 24, 2930-2939 (2007); T. Shay, V. Benham, J. T. Baker, A. D. Sanchez, D. Pilkington, and C. A. Lu, IEEE J. Sel. Top. Quantum Electron. 13, 480 (2007)]. The coherent combination has been shown in continuous and almost-continuous regime, and recently in femtosecond regime [cf. the publications L. Daniault, M. Hanna, L. Lombard, Y. Zaouter, E. Mottay, D. Goular, P. Bourdon, F. Druon, and P. Georges, “Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers,” Opt. Lett. 36, 621-623 (2011) and Enrico Seise, Arno Klenke, Jens Limpert, and Andreas Tünnermann, “Coherent addition of fiber-amplified ultrashort laser pulses,” Opt. Express 18, 27827-27835 (2010)].
However, the active devices for coherent recombination are complicated because they need a real-time feedback electronic system whose implementation is difficult and expensive.
The power increase of the active devices for coherent recombination toward higher energies and/or average powers thus remains problematic.